Not Applicable
Not Applicable
Not Applicable
The present invention relates to torque sensors and particularly transducers for providing an electrical signal indicative of the changes in torsional strain which a torsion member undergoes when subjected to a varying applied torque or moment. Such sensors and particularly transducers for sensing torsional strain are particularly desirable in systems where a shaft is subjected to varying torque during operation of the system including static, quasi-static and dynamic torque variation. Such applications include steering shafts for motor vehicles and rotor shafts in motors or power transmission shafts.
Heretofore, torque transducers for shafts have employed magneto elastic elements positioned on the shaft and sensing coils for detecting the change in the magnetic field as the magneto elastic element has been subjected to torsional strain. However, the difficulties in attaching the magneto elastic element to the shaft in a manner which could reliably transmit the shaft strain to the magneto elastic element created problems which rendered mass production of the device prohibitive.
A known transducer employing a piezoelectric element is that employing a piezoelectric element mechanically constrained and electrically connected to receive the output of the piezoelectric element upon the transducer being subjected to high impulse loads. Such a device is shown and described in U.S. Pat. No. 4,835,436 issued to H. S. Lew and is unsuitable for applications where a low torsional strain rate and/or low magnitude torque is applied to the torsion member, and is particularly not suitable for automotive steering shaft applications.
In quasi-static applications such as for an automotive steering shaft, it has further been desired to provide an electrical indication of the angular movement of the shaft; and, heretofore this has required a separate angle position sensor.
Thus it has long been desired to provide a way or means of sensing and providing an electrical indication of torsional strain in a member subjected to an applied torque in a manner that provides a high degree of sensitivity and relatively high resolution of the electrical signal output in response to changes in the torsional strain on the member. It has further been desired to provide such a torque sensor or transducer which provides an electrical indication of rotary angular position, which is easy to manufacture and install in high volume mass production and which is robust in service and relatively low in manufacturing cost.
The present invention provides a torque sensor or transducer which provides a continuous electrical indication of the torsional strain which a member undergoes when subjected to an applied torque. The transducer of the present invention employs a piezoelectric element potted onto a prepared area of the torsion member to be strained; and, the piezoelectric element is capacitively coupled via a rotary capacitor to an external detection circuit employing an oscillator. The rotary capacitor has a variable capacitance for indicating the angular deflection thereby eliminating the need for a separate angular position sensor. The piezoelectric element has the electrodes thereof covered and resilient dielectric spacers provide an air gap between the electrodes and the cover forming a subassembly to improve the signal output of the sensor at its resonant frequency. The piezoelectric element subassembly is secured to the prepared area preferably a recess, by a strain transmitting potting medium. In the presently preferred practice, the detection circuit includes an oscillator and detects changes in the resonant frequency of the piezoelectric element as a measure of the strain to which the torsion member is subjected.
The present invention provides a simple to manufacture and robust piezoelectric torque transducer which is relatively low in cost and easy to manufacture in high volume production and is particularly suitable for attachment to a quasi-statically strained element as, for example, a vehicle steering shaft or to a rotating shaft such as a rotating power transmission shaft or motor shaft.